1. Field of the Invention
The present invention relates to a display optical system, and in particular, though not exclusively, to a display optical system configured to be incorporated in a head mounted display or in a liquid crystal projector or included in an image display apparatus.
2. Description of the Related Art
A head mounted display includes an image display element, such as a cathode-ray tube (CRT) or a liquid crystal display (LCD), and an optical system capable of enlarging an image displayed on the image display element.
The head mounted display is generally compact and lightweight. In particular, the head mounted display is required to be thin enough in a direction parallel to the visual axis of an observer to assure a desirable weight balance and good appearance when mounted on the head of the observer.
Furthermore, the head mounted display is required to display a sufficiently large size of image so as to include strong visuals.
To satisfy such requirements, there is a one-time image forming display optical system which uses a decentered reflecting surface.
As discussed in Japanese Patent Application Laid-Open No. 10-153748, a display optical system includes a prism body having decentered reflecting surfaces and a relay lens unit. Light from an image displayed on an image display element enters the prism body via the relay lens unit and forms an intermediate image therein so that an observer can view the image displayed on the image display element.
As discussed in U.S. Pat. No. 7,012,756, a prism body includes three different decentered reflecting surfaces, between which a light flux can repeatedly reflect to provide a long optical path length capable of forming an intermediate image in a compact optical system. Thus, the overall size of the display optical system becomes compact, and an image having a wide angle of view can be displayed with a compact image display element.
A display optical system using a decentered reflecting surface is useful in that a compact body is realized and higher optical characteristics can be easily attained.
However, the optical characteristics may easily deteriorate if optical elements (e.g., a prism body or lens) and a lens barrel of the display optical system have fabrication or manufacturing errors. In particular, a decentered reflecting surface requires higher decentering accuracy not comparable to that for a refracting surface.
For example, if a prism body having decentered reflecting surfaces discussed in Japanese Patent Application Laid-Open No. 10-153748 or U.S. Pat. No. 7,012,756 has errors in relative position, various aberrations including decentration coma and astigmatism will be generated and, as a result, the resolution will deteriorate and desired optical characteristics cannot be obtained.
Therefore, to satisfy requirements for higher decentering accuracy, optical elements and lens barrels are strictly required to have extremely higher fabrication accuracy. Thus, the manufacturing cost significantly increases. The optical system discussed in Japanese Patent Application Laid-Open No. 10-153748 or U.S. Pat. No. 7,012,756 may not be suitable for mass production.